In the manufacture of preforms from which optical fibers may be drawn that are suitable for use as telecommunication lightguides, vapors of materials, such as SiCl.sub.4, are entrained in an oxidizing carrier gas such as oxygen. The vapor stream is then flowed into a rotating glass substrate tube which is made of quartz, for example, and which is being traversed by a torch. As the vapor stream encounters a zone of heat generated by the torch, oxide particles are formed which deposit on and fuse to the interior wall of the tube. After a predetermined number of oxide layers have been built up in the tube to form a preform tube, the tube, except for its end portions, is collapsed into a solid preform rod, from which fiber may be drawn. The hereinbefore described process is generally known as the modified chemical vapor deposition or MCVD process. For purposes of this discussion, both the substrate and the preform tubes will be referred to as the preform tube.
In the MCVD process, the tube is mounted in a lathe with each tube end held in a rotatable chuck. In order to inhibit the vapor stream from leaking to ambience and ambient air from entering and thereby contaminating the vapor stream with water vapor, a rotary seal is employed at the juncture of stationary and rotary conduits of the lathe through which the vapor stream is introduced into the preform tube. Additionally, oxygen, a component of the vapor stream itself, is often flowed under pressure over an end portion of the preform tube near the tube inlet as an added precaution to inhibit ambient air from entering the tube.
At the lathe headstock end of the preform tube, a rotating conduit which is connected to a source of material used in deposition must be connected to an end portion of the glass preform tube in which the materials are to be deposited. Generally, an end portion of the conduit is received in one end of a passageway which extends through a connecting member or is integral therewith. An end portion of the glass preform tube is received in the passageway at the other end of the connecting member. The diameter of the passageway at the other end of the connecting member is larger than that of the predorm tube to permit a plurality of non-metallic ferrules to be disposed about the end portion of the preform tube which is received in the passageway of the connecting member. Metallic rings are interposed between the non-metallic bands. A clamping nut having internally disposed threads at one end is turned over an externally threaded extension of the connecting member. As it is turned onto the connecting member, the clamping nut causes the ferrules to be compressed into sealing and gripping engagement with the end portion of the preform tube and a sealing surface which defines the passageway hereinafter referred to as the passageway sealing surface.
Non-metallic ferrules are used to ensure a sealed engagement with the preform tube. Because of the material of which they are made, that is, typically glass-filled TEFLON.RTM., an E.I. duPont de Nemours and Company polymer product, the ferrules are compressed into compliant engagement with the preform tube and the passageway sealing surface.
With the heat energy which is applied to the preform tube during a depositon mode and during a collaspe mode, the ferrules soften and undergo stress relaxation. This results in creep of the ferrule material. Although there would be some creep because of the difference in temperature during installation and subsequent ambient temperature, the creep is aggravated because of the relatively high temperatures used during the deposition mode and during the collapse mode.
When the ferrules undergo softening and creep, the engagement of the ferrules with the tube and the passageway sealing surface becomes less than satisfactory. Portions of the materials which are in vaporous form and which are being flowed through the tube supply conduit may escape and contaminants from the ambient atmosphere may enter the conduit. Also, when the opposite end portion of the preform tube or rod is subjected to the zone of heat, any gripping of that end portion at that end is diminished. This in effect causes the preform tube or rod to behave as a cantilever beam. For the tube or the rod at this stage to have adequate support, the ferrules must provide a suitable gripping engagement with the end portion of the preform tube or rod.
Accordingly, because of the creep of the ferrules, steps must be taken to maintain the sealed, gripping engagement of the ferrules with the preform tube and the passageway sealing surface. This is accomplished by an operator, generally at the end of the deposition mode and prior to collapse, who causes the clamping nut to be tightened.
The tightening of the clamping nut may cause another problem. Typically, abutting faces of adjacent ferrules and rings are beveled. As the clamping nut is tightened, it is not uncommon for the thinnest portion of a ferrule to be extruded between an adjacent metallic annular portion and the preform tube. The reconfiguring of a ferrule to accommodate such extrusion may cause its beveled surface to become irregular thereby reducing the area for pressure contact with a threaded extension of the connecting member over which the clamping nut is turned.
Also, as the nut is tightened and portions of at least the ferrule closest to the tailstock are extruded, resistance builds to further extrusion. Then as the nut may be further tightened with reactive forces being applied to the inner thickest portion of the ferrule, forces are caused to be applied by a center portion of the ferrule radially inwardly against the tube. This may result in breakage of the tube. Not only is the preform tube itself relatively expensive, but the breakage just prior to collaspe, which is when the tightening typically occurs, results in the scrapping of a preform tube in which the deposition has taken place and hence in which there is substantial investment.
Another problem occurs because the ferrules are made of a compliant, non-metallic material. A compliant seal with the tube is not achieved with a metallic ferrule. Because the coefficient of expansion of the metal bands and connecting member as well as that of the nut is greater than that of the glass preform tube, the metal portions during the heating process tend to move away from the ferrules thereby resulting in a loosening of the joint between the preform tube and the connecting member.
Further, in current MCVD processes, one technique involves the use of a roller which is caused to engage the rotating preform tube to straighten the preform tube before fiber is drawn therefrom. It has been found that maintenance of the clamping or gripping force at the connection of the preform tube to the supply conduit is critical during this operation, otherwise, distortion of the tube could occur.
In order to keep the clamping nut as cool as possible in an attempt to minimize ferrule creep, the clamping nut is somewhat enlarged to allow the provision of cooling fins. A prior art rotary connector with its cooling fins occupy a portion of the length of the preform tube which becomes unusable. Bearing in mind that each fraction of length of the preform tube represents a substantial investment, it would be most beneficial to reduce that length of the preform tube over which the connector extends.
What is needed and what seemingly is not available in the prior art is a connector which is self-tightening during its use. The sought-after connector should be usable with any tube to connect the tube with another tube and should be capable of being incorporated easily into a lathe which is used in the MCVD process by retrofitting the lathe.